Soot preforms are commonly used to form optical fiber glass preforms in optical fiber manufacturing. Making optical fiber preforms that are free of defects, and that can be readily processed, is commonly a time consuming process. While conventional processes for forming soot preforms result in the deposition of generally porous layers of soot, some portions of a resulting soot preform tend to be less porous than other portions of a resulting soot preform. The flow of processing gases, which are contacted with a soot preform during various processing steps, through the less porous portions of the soot preform may be limited as compared to the flow of processing gases through the porous portions of the soot preform.
Conventionally, forming a consolidated glass preform that is free of voids, trapped gas bubbles, or other defects includes sintering soot preforms very slowly in a furnace at approximately atmospheric pressure. As the soot preform is sintered, the pores transition from open pores to closed pores, and sintering rates must be reduced so that gas does not become trapped in the closed pores. Even after the pores have been closed, additional processing is completed to diffuse the sinter gas out of the glass preform. Diffusion of the gas avoids the formation of gas seeds at high temperatures associated with drawing the consolidated glass preforms into optical fiber. Diffusion of gas may be of particular interest when preforms are bulk sintered. During bulk sintering, the entire preform is heated to temperatures within a sintering temperature range, and, generally, the outermost portions of the preform densify first. Additionally, the less porous portions of the soot preform tend to densify before the porous portions of the soot preform. Once densified, these portions of the soot preform act as a barrier and minimize paths for the diffusion of gas out of the preform.
While some reduction in the sintering period may be accomplished by using a high permeability sinter gas such as, but not limited to, helium (in contrast to low permeability gases such as, but not limited to, nitrogen, argon and oxygen), the sintering period remains relatively long in order to ensure that the helium does not become trapped in the consolidated glass preform.
Conventionally, diffusion of the gas may be accomplished by placing sintered glass preforms in holding ovens at temperatures ranging from about 800° C. to about 1,200° C. to diffuse interstitial gas that becomes dissolved in the glass perform during sintering. While the period in a holding oven for interstitial gas to diffuse out of the glass preform increases with increasing glass preform size, it is common for the glass preform to remain in the holding oven for at least more than 1.0 hour.
The combination of the time to perform steps in the manufacture of optical fiber glass preforms results in very long processing times for the optical fiber preforms, which negatively impacts the cost of manufacturing optical fiber.